A single rotary dryer (Kiln) or a three-pass rotary dryer is an industrial drier used for multiple purposes to dry sludge or food waste. In general, sludge has the percentage of water content approximating 80%. After passing through a dryer, the sludge has the drying performance of less than 10%.
The three-pass rotary dryer represents superior drying efficiency 1.5 times greater than that of the single rotary dryer. The three-pass rotary dryer had been registered as a patent in U.S. for the first time in 1984 (see cited reference 1). In U.S., several companies (Vincent Co. and Stela Co.) have developed and used the three-pass rotary dryer in order to dry various foods or coffees. Recently, as environmental pollution has been issued, the three-pass rotary dryer has been used for drying swage sludge (e.g., ANDRITZ Co. of Austria). However, since the drying efficiency and the circulation rate are lowered in the 2nd or 3rd drum of the dryer, it is not easy to obtain the dried substance with the proper percentage of water content. In addition, since the internal circulation space for the sludge is relatively narrow, the sludge may be massed, so that the rotary dryer may be clogged. If the amount of the food waste or the size of a target to be dried is great, the existing three-pass rotary dryer cannot be used. In order to reduce the clogging phenomenon, the external diameter of the drum must be increased. However, a three-pass rotary dryer having a large diameter requires a wider installation area, represents energy inefficiency, and is dangerous in operation thereof.    Cited Reference 1: U.S. Pat. No. 4,477,984 (Oct. 23, 1984), Harvey Wenger